Neural Injury and Repair in a Novel Neonatal Mouse Model of Listeria Monocytogenes Meningoencephalitis.

To improve the therapy of neonatal central nervous system infections, well-characterized animal models are urgently needed. The present study analyzes neuropathological alterations with particular focus on neural injury and repair in brains of neonatal mice with Listeria monocytogenes (LM) meningitis/meningoencephalitis using a novel nasal infection model. The hippocampal formation and frontal cortex of 14 neonatal mice with LM meningitis/meningoencephalitis and 14 uninfected controls were analyzed by histology, immunohistochemistry, and in situ tailing for morphological alterations. In the dentate gyrus of the hippocampal formation of mice with LM meningitis/meningoencephalitis, an increased density of apoptotic neurons visualized by in situ tailing (p = 0.04) and in situ tailing plus immunohistochemistry for activated Caspase-3 (p < 0.0001) was found. A decreased density of dividing cells stained with an anti-PCNA-antibody (p < 0.0001) and less neurogenesis visualized by anti-calretinin (p < 0.0001) and anti-calbindin (p = 0.01) antibodies were detected compared to uninfected controls. The density of microglia was higher in LM meningitis (p < 0.0001), while the density of astrocytes remained unchanged. Infiltrating monocytes and neutrophilic granulocytes likely contributed to tissue damage. In conclusion, in the brains of LM-infected mice a strong immune response was observed which led to neuronal apoptosis and an impaired neural regeneration. This model appears very suitable to study therapies against long-term sequelae of neonatal LM meningitis.

Interleukin-1 receptor type 1 is essential for control of cerebral but not systemic listeriosis.

Listeria monocytogenes may infect the central nervous system and several peripheral organs. To explore the function of IL-1 receptor type 1 (IL-1R1) in cerebral versus systemic listeriosis, IL-1R1(-/-) and wild-type mice were infected either intracerebrally or intraperitoneally with L. monocytogenes. After intracerebral infection with various numbers of attenuated Listeria, IL-1R1(-/-) mice succumbed due to an insufficient control of intracerebral Listeria, whereas all wild-type mice survived, efficiently restricting growth of Listeria. IL-1R1(-/-) mice recruited increased numbers of leukocytes, especially granulocytes, to the brain compared with wild-type mice. In contrast, both IL-1R1(-/-) and wild-type mice survived a primary and secondary intraperitoneal infection with Listeria without differences in the hepatic bacterial load. In addition, both strains developed similar frequencies of Listeria-specific CD4 and CD8 T cells after primary and secondary intraperitoneal infection. However, an intraperitoneal immunization before intracerebral challenge infection neither protected IL-1R1(-/-) mice from death nor reduced the intracerebral bacterial load, although numbers of intracerebral Listeria-specific CD4 and CD8 T cells and levels of inducible nitric oxide synthase, tumor necrosis factor, and interferon-gamma mRNA were identical in IL-1R1(-/-) and wild-type mice. Collectively, these findings illustrate a crucial role of IL-1R1 in cerebral but not systemic listeriosis.

TNF is important for pathogen control and limits brain damage in murine cerebral listeriosis.

Cerebral listeriosis is a life-threatening disease. However, little is known about the bacterial virulence factors responsible for the severe course of disease and the factors of the immune system contributing to the control of Listeria monocytogenes (LM) or even to the damage of the brain. To analyze the importance of the actA gene of LM, which mediates cell-to-cell spread of intracellular LM, the function of TNF in murine cerebral listeriosis was studied. C57BL/6 mice survived an intracerebral (i.c.) infection with actA-deficient LM, but succumbed to infection with wild-type (WT) LM. Upon infection with actA-deficient LM, macrophages and microglial cells rapidly, and later LM-specific CD4 and CD8 T cells, produced TNF. In contrast to WT mice, TNF-deficient animals succumbed to the infection within 4 days due to failure of control of LM. Histology identified a more severe meningoencephalitis, brain edema, and neuronal damage, but a reduced inducible NO synthase expression in TNF-deficient mice. Reciprocal bone marrow chimeras between WT and TNF-deficient mice revealed that hematogenously derived TNF was essential for survival, whereas TNF produced by brain-resident cells was less important. Death of TNF-deficient mice could be prevented by LM-specific T cells induced by an active immunization before i.c. infection. However, brain pathology and inflammation of immunized TNF-deficient mice were still more severe. In conclusion, these findings identify a crucial role of TNF for the i.c. control of LM and survival of cerebral listeriosis, whereas TNF was not responsible for the destruction of brain tissue.

Neuropathological findings in 9 cases of listeria monocytogenes brain stem encephalitis.

Brain stem encephalitis is a particular manifestation of infection with the bacterium Listeria monocytogenes. Here, we present the neuropathological findings in 9 such cases. In the brain stem, the inflammatory infiltrates were located predominantly within nuclei and tracts of cranial nerves innervating the oropharynx. These findings support the hypothesis that the food-borne bacterium Listeria monocytogenes invades the brain stem along cranial nerves.

Expression of E-selectin, P-selectin, and intercellular adhesion molecule-1 during experimental murine listeriosis.

The expression of adhesion molecules E-selectin, P-selectin, and intercellular adhesion molecule-1 (ICAM-1) was immunohistochemically investigated during the course of experimental murine listeriosis. Infection was monitored by microbiological count of blood, liver, and spleen. After an early generalized expression of P-selectin and ICAM-1, a later regulation occurred specifically to areas of inflammation. Expression of E-selectin was faint and inconstantly detected in all of the studied organs. In the liver, typical lesions of murine listeriosis were related to the expression of ICAM-1 on sinusoidal endothelial cells and the biliary system and to the de novo expression of P-selectin in hepatic portal vessels. Inflammation in the spleen was related to the expression of ICAM-1 on red pulp sinusoidal cells, especially in the marginal sinus. High endothelial venules of inflamed lymph nodes also expressed P-selectin and ICAM-1. Lesions in the central nervous system appeared on day 3 after infection as a pyogranulomatous leptomeningitis associated with an intense expression of P-selectin and ICAM-1 in meningeal vessels, especially those in the hippocampal sulcus, suggesting a way through which inflammation initially reach the central nervous system during experimental murine listeriosis. Leptomeningitis was followed by the presence of ventriculitis, which was related to the up-regulation of ICAM-1 on choroid plexus epithelial cells, periventricular vessels and ependymal cells. Up-regulation of P-selectin and ICAM-1 during experimental murine listeriosis could play an important role in the recruitment of leukocytes, especially to the liver, lymphoid organs, and central nervous system.

Experimental Listeria meningoencephalitis. Macrophage inflammatory protein-1 alpha and -2 are produced intrathecally and mediate chemotactic activity in cerebrospinal fluid of infected mice.

In bacterial meningitis, the recruitment of leukocytes across the blood-brain barrier into the central nervous system may be crucial for both elimination of pathogens and tissue injury. In addition to bacterial cell wall products, host factors including chemokines may lead to accumulation of phagocytes within the central nervous system. As shown by Northern analysis, brains of mice infected intracerebrally with Listeria monocytogenes (LM) express mRNA for three chemokines, the macrophage inflammatory protein (MIP)-1 alpha, MIP-1 beta, and MIP-2. The cellular sources of these chemokines comprise both the blood-derived polymorphonuclear leukocytes (PMNs) and monocytes infiltrating the meninges, the ventricular system and the periventricular area. In the course of meningitis a time-dependent increase of MIP-1 alpha and MIP-2 was found in the cerebrospinal fluid (CSF) by ELISA. CSF taken 24 h after infection (CSF-LM24) induced migration of human leukocytes when treated in chemotactic chambers in vitro. Neutralizing Abs to chemokines identified MIP-1 alpha and MIP-2 to be responsible for CSF-LM24 mediated chemotaxis of monocytes and PMNs, respectively. CSF obtained from mock-infected animals contained no MIP-1 alpha or MIP-2 and did not lead to migration of leukocytes. When testing CSF-LM24 on mouse spleen cells, the chemotactic activity detected for mononuclear cells was only partly inhibited by Abs to MIP-1 alpha and -1 beta. Thus, in addition to MIP-1 and -2 other not yet defined chemotactic factors are of importance for recruitment of leukocytes in bacterial meningitis.